I. Technical Field
The present invention mainly relates to a light emitting device comprising a flip-chip mounted GaN-based LED chip with a light emitting element structure consisting of a GaN-based semiconductor formed on a light-transmissive substrate.
II. Description of the Related Art
The GaN-based semiconductor is a compound semiconductor represented by a chemical formula:AlaInbGa1-a-bN (0≦a≦1, 0≦b≦1, 0≦a+b≦1),which is also called a III group nitride semiconductor, a nitride-based semiconductor and the like. A compound semiconductor of the above-mentioned chemical formula wherein a part of the III group element is substituted by B (boron), Tl (thallium) and the like and one wherein a part of N (nitrogen) is substituted by P (phosphorus), As (arsenic), Sb (antimony), Bi (bismuth) and the like are also encompassed in the GaN-based semiconductor. A GaN-based LED wherein a light emitting element structure such as a pn junction structure, a double heterostructure and a quantum well structure is constituted of a GaN-based semiconductor can emit green to near ultraviolet light, and has been put to a practical use for signal, display apparatus and the like.
A GaN-based LED chip comprising a GaN-based semiconductor layer with a light emitting element structure formed on a light-transmissive substrate can be fixed on a base such as a substrate of an SMD (Surface Mount Device)-type LED package and a lead frame, directly or via a submount, with the GaN-based semiconductor layer side facing the base. In other words, the LED chip can be fixed on a base with the light-transmissive substrate side facing upward. This type of chip bonding form is called flip-chip mounting. The flip-chip mounting is also sometimes called facedown mounting, upside-down mounting, junction-down mounting and the like. Conventional GaN-based LED chips for flip-chip mounting have, on a surface of a GaN-based semiconductor layer and the like, a metal reflection film also acting as an electrode, so as to reflect light generated inside the GaN-based semiconductor layer to the light-transmissive substrate side. To improve the emission power when mounted, it is considered desirable to form this reflection film using a metal having a high light-reflection coefficient such as Ag (silver), Al (aluminum) and Rh (rhodium) as a material (JP-A-2000-183400, JP-A-2004-179347). See also, JP-A-2002-280611 and JP-A-2003-318441.